There are some 40 million e-readers and 65 million tablets in use in the U.S., according to analysts at Forrester Research. In the first quarter of 2012, e-books generated $282 million in sales, compared with $230 million for adult hardcover books, the Association of American Publishers recently found.

Meanwhile, the shift to digital books has fueled an arms race among digital start-ups seeking to cash in on the massive pool of data collected by e-reading devices and reading apps. New e-reading services, which allow readers to purchase and store books in a digital library and read them on different devices, have some of the most sophisticated reader tracking software. The digital reading platform Copia, which has 50,000 subscribers, collects detailed demographic and reading data—including the age, gender and school affiliation of people who bought particular titles, as well as how many times the books were downloaded, opened and read—and shares its findings with publishers. Copia aggregates the data, so that individual users aren't identifiable, and shares that information with publishers that request it.